1. Field of the Invention
The present invention relates to a control coupling for torque transmission between a pair of rotary members coaxially arranged for relative rotation, and more particularly to a control coupling disposed in a drive system of a motor vehicle for torque transmission.
2. Description of the Related Art
This kind of control coupling is used as a torque transmission mechanism for effecting drive connection between a pair of rotary shafts coaxially arranged for relative rotation and is also used as a limited-slip differential for limiting relative roatation between a pair of coaxially arranged drive and driven members. In the former use, the control coupling is disposed in a torque transmission system of a four-wheel drive vehicle. In the latter use, the control coupling is assembled within a differential unit.
Disclosed in Japanese Patent Early Publication No. 63-240429 is a control coupling for torque transmission between a pair of rotary shafts coaxially arranged for relative rotation. The coupling includes a cylindrical casing mounted on one of the rotary shafts for rotation therewith, a multiple friction disc clutch assembled within the casing for effecting drive eonnection between the rotary shafts when it has been engaged, and a thrust mechanism assembled within the casing for producing a thrust force in accordance with a difference in rotation between the shafts and applying it to the friction disc clutch. The thrust mechanism includes a thrust piston axially slidably coupled within the casing in a fluid-tight manner for rotation therewith and being in engagement with the friction disc clutch, an end wall member coupled within the casing to rotatably support thereon one end of the other rotary shaft and spaced from the piston to form a fluid chamber filled with an amount of viscous fluid, and a rotor disposed in the fluid chamber and mounted on the other rotary shaft for rotation therewith, the rotor being integrally formed with a plurality of circumferentially spaced radial blades.
In operation of the control coupling, relative rotation of the rotor to the piston and end wall member causes the viscous fluid to flow circumferentially in a plurality of chambers subdivided by the radial blades. This produces an internal pressure acting on the friction disc clutch in accordance with a difference in rotation between the rotary shafts. Thus, the friction disc clutch is engaged by the internal pressure applied thereto to effect drive connection between the rotary shafts. In this instance, the engagement force of the clutch is controlled in proportion to the difference in rotation between the rotary shafts.
In the control coupling described above, the internal pressure of viscous fluid is produced in inverse proportion to the cube of the width of the fluid chamber in an axial direction. For this reason, it is desirable that the fluid chamber is formed as small as possible in width to increase the internal pressure of viscous fluid. In the case, however, that the fluid chamber is reduced in width, the amount of stored viscous fluid becomes small since the fluid chamber may not be enlarged in diameter due to size limitation of the control coupling. This causes the following problems:
(a) The viscous fluid is deteriorated in a short period of time due to thermal and mechanical influences.
(b) The internal pressure is greatly influenced by a difference in the amount of viscous fluid filled in the fluid chamber. It is, therefore, difficult to uniformly determine the torque transmission characteristic of the control coupling.